The skin mainly consists of the epidermis, the dermis, and the subcutaneous tissue. The dermis is located beneath the epidermis and is filled with the biological structure called the extracellular matrix constituted of, for example, collagen and hyaluronic acid for supporting the skin structure. The extracellular matrix components are produced by, for example, fibroblasts. In the skin maintaining its youthfulness, production of the extracellular matrix components is accelerated to keep elasticity, moisture, and tension of the skin. It is said that, in contrast, aging of the skin, represented by symptoms such as pigmented spots, wrinkles, sagging skin, and rough skin, progresses with a reduction in extracellular matrix components such as hyaluronic acid.
Hyaluronic acid is one of glycosaminoglycans and is a polymer having a chain structure where glucuronic acid and N-acetyl glucosamine residues are repeatedly linked without having sulfate groups.
Hyaluronic acid is distributed widely throughout the body, for example, in the skin, tendon, muscle, cartilage, brain, and blood vessels. The hyaluronic acid, a main structural component of the extracellular matrix, is known to be present between cells to retain an extracellular fluid and show, for example, a wound healing effect and a joint lubricating effect (Development of functional glyco-material and application to foods, 2005, pp. 324-325). It has been reported that the amount of hyaluronic acid in human skin decreases with aging, and it is supposed that the decrease of hyaluronic acid in the skin is one cause of a reduction in skin elasticity and a decrease in moisture due to aging (Carbohydrate Research, 159(1), 127-136, 1987; International Journal of Dermatology, 33(2), 119-122, 1994).
In order to improve the state due to a decrease in hyaluronic acid, various materials having effects of promoting hyaluronic acid production have been proposed. Examples thereof include retinoic acid, a Ptychoverpa bohemica extract (Japanese Patent Laid-Open No. 2008-222664), a saffron extract (Japanese Patent Laid-Open No. 2005-206510), an extract of seaweed belonging to the genus Durvillea of the family Durvillea (Japanese Patent Laid-Open No. H9-176036), and a peptide having a specific sequence (Japanese Patent Laid-Open No. 2007-145845).
The retinoic acid is known to be effective, but causes side effects such as dermatitis, and thereby safety issues have been concerned. In plant-derived materials such as the Ptychoverpa bohemica extract and the saffron extract or the seaweed extract of the genus Durvillea of the family Durvillea, the effects of promoting hyaluronic acid production are not high. With the peptide, it is necessary to intake a large amount of the peptide for obtaining the effect, and thus a satisfactory effect is hardly obtained.
Meanwhile, it is known that soybean isoflavone aglycones promote hyaluronic acid production (Japanese Patent Laid-Open No. 2001-335454). Furthermore, it is known that 20-O-β-D-glucopyranosyl-20(S)-protopanaxadiol (compound K), the main metabolite of ginseng saponin, increases expression of a hyaluronic acid synthase gene to promote hyaluronic acid production (PCT Japanese Translation Patent Publication No. 2006-515303). However, soyasapogenol has not been reported to promote hyaluronic acid production.
There are many unclear points in causes that cause skin pigmentation, such as pigmented spots and freckles, but it is supposed that one cause is enhancement in melanin synthesis function of melanocytes present in the basal epidermal layer, which is triggered by, for example, ultraviolet light or hormone imbalance. When melanocytes are stimulated by stimulus against epidermal cells (keratinocytes), tyrosine in the melanocytes is converted into dopa by action of, for example, tyrosinase, and dopa is further converted into dopaquinone and then into melanin. The synthesized melanin is accumulated in vesicles called melanosome. Melanosomes containing the accumulated melanin are supplied to keratinocytes from melanocytes.
For preventing and improving the above-described skin pigmentation, various materials that inhibit melanin synthesis to achieve whitening effect have been proposed, and examples thereof include ascorbic acid, glutathione, and kojic acid (Japanese Patent Laid-Open No. S53-3538), an Illicium verum extract (Japanese Patent Laid-Open No. H11-302149), conagenin (PCT Japanese Translation Patent Republication No. 2007-011066), and acidic oligosaccharide (Japanese Patent Laid-Open No. 2009-13084).
Unfortunately, the ascorbic acid is easily oxidized and therefore has low storability. The glutathione has a drawback of causing a particular off-flavor. The kojic acid has a problem of safety in use at a high concentration. The Illicium verum extract, conagenin, and acidic oligosaccharide need high concentrations for showing their effects, and it is not concerned that satisfactory effects are achieved.
Meanwhile, it has been reported that soybean isoflavone inhibits melanin synthesis (Japanese Patent Publication No. S60-19885). It is also known that 2,3-dihydro-3,5-dihydroxy-6-methyl-4H-pyran-4-one and 2,3-dihydro-3,5-dihydroxy-6-methyl-4H-pyran-4-one-binding saponin (DDMP-saponin) inhibits melanin synthesis (Japanese Patent Laid-Open No. 2006-28077). However, the melanin synthesis-inhibiting effects of soybean saponins, other than the DDMP-saponin, and soyasapogenols have not been reported.